New MRSA strain appears in cows

A new strain of methicillin-resistant Staphylococcus aureus, commonly known as MRSA, has been discovered in cows and humans in the United Kingdom and elsewhere in Europe, researchers reported Thursday. The new strain disturbs researchers because it evades one of the most commonly used tests to detect MRSA, which could lead physicians to prescribe the wrong antibiotics to treat the infection. The new strain of the bacterium is still relatively rare and, so far, no deaths have been attributed to it, the team reported in the journal Lancet Infectious Diseases. Its discovery in cows raises a new question about the origin of MRSA outbreaks, however: Are cows a natural reservoir for the infections or are they infected by humans who come into contact with them?

The presence of the bacteria in cows does not present a threat to the food supply because it is killed during the pasteurization process. But the infection can be transmitted to humans who come into close contact with the animals, and these workers can then pass the bacteria into the general population.

Although MRSA infections may be declining in the United States, they still represent a serious healthcare problem, with an estimated 90,000 new infections linked to healthcare facilities each year and about 15,000 deaths, mostly in older people or those with underlying health problems. Staph infections normally reside in the nose and are generally benign, but invasive staph infections can spread to the blood, lungs, soft tissues, bones and joints.

Laura Garcia-Alvarez, a post-doctoral fellow at the University of Cambridge’s Veterinary School in England discovered the new strain while collecting samples from cattle in the United Kingdom. S. aureus is a common cause of mastitis, an inflammation of the udders, in cows. When she swabbed samples onto petri dishes impregnated with antibiotics related to methicillin, the bacteria grew, indicating that it was resistant. But a rapid PCR test, which is increasingly being use to detect MRSA, was negative. The PCR tests are becoming widely used because they take only 30 minutes, compared to two to three days for growing the bacteria on a petri dish.

Methicillin resistance is commonly conferred by a specific variant of the mecA gene, and it is this variant that the PCR analysis tests for. When geneticist Matt Holden of the Wellcome Trust Sanger Institute in Britain analyzed the DNA sequence of the bacterium, however, he found that it contained a different variant of mecA. This variant does not respond in the conventional PCR test.

The new variant has been linked to infections of humans in the U.K. and elsewhere in Europe, the team reported, although the numbers are still small, probably less than 100 cases per year in the U.K. “It does appear that the numbers are rising,” however, said veterinarian Mark Holmes of the University of Cambridge, the senior author of the report. “The next step will be to explore how prevalent the new strain actually is and to track where it is coming from. If we are ever going to address the problem with MSRA, we need to determine its origins.” Some researchers have speculated that the bacterium’s presence in cattle may arise from overuse of antibiotics in the animals.

There is no evidence that the new strain of MSRA has appeared in the United States, but researchers have not been looking for it, so it could be residing in dairy animals undetected.